1. Field of the Invention
The present invention relates to a microstrip line filter and a high-frequency transmitter using the microstrip line filter. In particular, the present invention relates to a microstrip line filter constituting a low-pass filter which eliminates any unwanted radiation and relates to a high-frequency transmitter using the microstrip line filter.
2. Description of the Background Art
In recent years, the radio (high-frequency) communication has undergone remarkable developments in numerous systems like the broadcast and communication satellites for example. On the other hand, the widespread use of the Internet has caused increasing demands for the two-way communication.
FIG. 11 schematically shows a system for two-way communication by means of a communication satellite. Referring to FIG. 11, an IDU (indoor unit) 1 is contained within a television receiver or housed in a board in a personal computer, and processes a signal for two-way communication with a broadcast station via a communication satellite 2. IDU 1 is connected to a high-frequency transmitter 4 via a transmission-adapted coaxial cable 3 and IDU 1 is also connected to an LNB (low noise block down converter) 6 via a reception-adapted coaxial cable 5.
High-frequency transmitter 4 and LNB 6 are coupled to a feed horn 8 via an orthogonal polarization isolator 7. A transmission signal from high-frequency transmitter 4 is radiated as the microwave from feed horn 8, reflected by a parabolic antenna 9 and transmitted toward communication satellite 2. The microwave from communication satellite 2 is reflected by parabolic antenna 9 and then received by LNB 6 via feed horn 8.
FIG. 12 is a block diagram of the high-frequency transmitter employed in the system shown in FIG. 11. Referring to FIG. 12, high-frequency transmitter 4 receives, from IDU 1 shown in FIG. 11, a transmission signal of an intermediate frequency ranging from 950 to 1450 MHz superimposed on a direct-current voltage. The intermediate-frequency signal is supplied via a high-pass filter (HPF) 401 to an IF amplifier 402 to obtain a gain, adjusted to a proper level by an attenuator 403, further amplified by an IF amplifier 404, and then supplied to a mixer 406 via a bandpass filter (BPF) 405.
A local oscillator 407 generates a local oscillator signal of 13.05 GHz which is provided via a buffer amplifier 408 to mixer 406. Mixer 406 combines the local oscillator signal of 13.05 GHz with the intermediate-frequency signal of 950-1450 MHz in order to convert the intermediate-frequency signal into a high-frequency signal of 14.0-14.5 GHz. The high-frequency signal supplied from mixer 406 is input to a half-wave bandpass filter 409 where any unwanted radiation component (spurious radiation component) of the high-frequency signal that is generated in mixer 406 is attenuated, and then amplified by two high-frequency amplifiers 410 and 411 to obtain a great gain.
The output from high-frequency amplifier 411 is supplied to a bandpass filter 412 where the amplified spurious component is attenuated, and then supplied to a driver amplifier 413 to obtain a further gain. The output from driver amplifier 413 is supplied to a reception-bandwidth noise filter 414 where any noise level in a reception frequency range is substantially reduced to a thermal noise level. Then, the high-frequency signal is converted by a power amplifier 415 to a signal of high power required for transmission to the satellite. The high-frequency signal from power amplifier 415 is provided to a reception-bandwidth noise filter 416 where the noise level in the reception frequency range that is increased from the thermal noise level due to the gain of power amplifier 415 is attenuated, and then the signal supplied via noise filter 416 from high-frequency transmitter 4 is radiated as the microwave from feed horn 8, reflected by parabola antenna 9 and transmitted toward communication satellite 2 that are shown in FIG. 11.
The DC voltage with the intermediate-frequency signal superimposed thereon is supplied via an inductor L to a power supply circuit 421. Inductor L prevents the intermediate-frequency signal from being input to power supply circuit 421. Power supply circuit 421 converts the supplied DC voltage into a predetermined voltage which is provided to a power supply sequence circuit 422. Then, the converted DC voltage is supplied to IF amplifiers 402 and 404, mixer 406, local oscillator 407, buffer amplifier 408, high-frequency amplifiers 410 and 411, driver amplifier 413 and power amplifier 415.
In high-frequency transmitter 4 shown in FIG. 12, the gain of IF amplifiers 402 and 404 and the degree or amount of attenuation by attenuator 403 are adjusted to prevent the output level from varying when the level of the input intermediate-frequency signal varies in the range from xe2x88x925 dBm to xe2x88x9225 dBm. Even if a high-level signal of approximately xe2x88x925 dBm is input, IF amplifiers 402 and 404 operate in a saturation region to distort the signal component in order to output the signal at a predetermined level. However, the distorted signal component generates harmonic components resulting in increase of spurious components.
Any spurious of 14.95-15.95 GHz generated in mixer 406 resultant from mixing of the input signal of twice the frequency of 950 MHz-1450 MHz and the local oscillator signal of 13.05 GHz differs from the output frequency range 14 GHz-14.5 GHz of high-frequency transmitter 4 merely by 450 MHz. Then, in order to reduce such a spurious, a microstrip filter as shown in FIG. 13 is used as the half-wave bandpass filter 409 shown in FIG. 12.
The microstrip filter shown in FIG. 13 includes a plurality of (e.g. 8) rectangular elements shifted so that respective halves of the longitudinal sides of respective elements are opposite to and in parallel with each other. This bandpass filter 409 has a passband of 14 GHz-14.5 GHz so as to attenuate an image-frequency signal of 11.6-12.1 GHz and a signal above 14.5 GHz. However, proper attenuation of the spurious of 14.95 GHz which is close to 14.5 GHz could be impossible.
FIG. 14 shows cutoff characteristics of a combination of half-wave bandpass filter 409 and high-frequency amplifiers 410 and 411. It is seen from FIG. 14 that the attenuation achieved by the cutoff characteristics is merely 11.9 dB, which means that an attenuation of 20 dB or more by half-wave bandpass filter 409 with its elements arranged as shown in FIG. 13 is extremely difficult. Even if attenuation of at least 20 dB is possible, it is impossible to make the cutoff characteristics more steeper.
One object of the present invention is to provide a microstrip line filter constituting a low-pass filter with a large out-of-band attenuation and a small in-band deviation, and to provide a high-frequency transmitter employing the microstrip line filter.
In summary, according to one aspect of the present invention, a microstrip line filter formed on a substrate includes a plurality of composite elements arranged in parallel with each other. The composite elements each include a rectangular microstrip line element, an input microstrip line and an output microstrip line that are formed on the substrate. The composite elements are connected to constitute a low-pass filter.
The rectangular microstrip line element has one longer side, the other longer side, one end and the other end. The input microstrip line is connected at the one end to the one longer side, and the output microstrip line is connected at the other end to the other longer side.
The composite elements adjacent to each other have respective input microstrip line and output microstrip line connected to each other and, the adjacent composite elements are symmetrical with respect to a center line between the input microstrip line and the output microstrip line connected to each other of the adjacent composite elements respectively.
Rectangular microstrip line elements of the composite elements differ in the length of longer side.
The rectangular microstrip line elements include outer microstrip line elements and inner microstrip line elements. The inner microstrip line elements have longer sides shorter than those of the outer microstrip line elements to obtain desired input/output impedance characteristics, in-band pass characteristics and out-of-band attenuation characteristics.
Microstrip line elements of the composite elements are arranged symmetrically with respect to a center line of the arrangement of the composite elements, and the microstrip line filter includes a metal casing having a partition on the center line and covering microstrip line elements of the composite elements.
Microstrip line elements of the composite elements have respective input microstrip lines and respective output microstrip lines that connect the microstrip line elements and that have respective widths selected to obtain desired input/output impedance characteristics, in-band pass characteristics and out-of-band attenuation characteristics.
A half-wave bandpass filter connected in series to the low-pass filter is further formed on the substrate.
The half-wave bandpass filter includes a plurality of rectangular microstrip line elements arranged in parallel with each other at predetermined intervals and inclined at a certain angle, and halves of respective longitudinal sides of the microstrip line elements are opposite to halves of respective longitudinal sides of adjacent microstrip line elements.
According to another aspect of the present invention, a high-frequency transmitter converts an intermediate-frequency signal into a high-frequency signal and transmits the high-frequency signal. The high-frequency transmitter includes a mixer circuit combining the intermediate-frequency signal with a local oscillator signal, a filter circuit connected to an output of the mixer circuit, and a high-frequency amplifier circuit connected to an output of the filter circuit. The filter circuit is formed on a substrate and includes a half-wave bandpass filter including a plurality of rectangular microstrip line elements that are arranged in parallel with each other at predetermined intervals and inclined at a certain angle, halves of respective longitudinal sides of the microstrip line elements being opposite to halves of respective longitudinal sides of adjacent microstrip line elements. The filter circuit further includes a low-pass filter including a plurality of composite elements arranged in parallel with each other and cascaded, the composite elements including respective rectangular microstrip line elements, respective input microstrip lines and respective output microstrip lines.
According to the present invention, the low-pass filter provides a large out-of-band attenuation and a small in-band deviation and accordingly has improved spurious elimination characteristics. Specifically, attenuation of at least 40 dB out of the passband above the higher limit of the passband is achieved all the time without deterioration in deviation within the passband and accordingly elimination of spurious above 14.95 GHz is possible.
In addition, the low-pass filter of the present invention has composite elements symmetrically arranged. Specifically, composite elements adjacent to each other are symmetrical with respect to the center line between respective input and output lines connected to each other. Accordingly, the low-pass filter occupies a minimum space as compared with composite elements that are simply cascaded.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.